<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>外部</title>
    <link rel="stylesheet" href="a.css">
</head>
<body>
<!--<style>div{color:blue;}</style> 这里面是内部引入的方法-->
<div>hello css</div>
<div>hello world</div>
</body>
</html>